Hidden magnetic transitions in thermoelectric layered cobaltite, [Ca${}_2$CoO${}_3$]${}_{0.62}$[CoO${}_2$]

Abstract

A positive muon spin rotation and relaxation (${\mu }^{+}$SR) experiment on [Ca${}_2$CoO${}_3$]${}_{0.62}$[CoO${}_2$], ({\sl i.e.}, Ca${}_3$Co${}_4$O${}_9$, a layered thermoelectric cobaltite) indicates the existence of two magnetic transitions at $\sim$ 100 K and 400 - 600 K; the former is a transition from a paramagnetic state to an incommensurate ({\sf IC}) spin density wave ({\sf SDW}) state. The anisotropic behavior of zero-field ${\mu }^{+}$SR spectra at 5 K suggests that the {\sf IC-SDW} propagates in the $a$-$b$ plane, with oscillating moments directed along the c-axis; also the {\sf IC-SDW} is found to exist not in the [Ca${}_2$CoO${}_3$] subsystem but in the [CoO${}_2$] subsystem. In addition, it is found that the long-range {\sf IC-SDW} order completes below $\sim$ 30 K, whereas the short-range order appears below 100 K. The latter transition is interpreted as a gradual change in the spin state of Co ions %% at temperatures above 400 K. These two magnetic transitions detected by ${\mu }^{+}$SR are found to correlate closely with the transport properties of [Ca${}_2$CoO${}_3$]${}_{0.62}$[CoO${}_2$].

7 pages, 8 figures. to be appeared in Phys. Rev. B